This invention relates to a nuclear magnetic resonance apparatus for measuring from outside of a body both the distribution of a specific atomic nucleus in the body to be examined and the relaxation time of the atomic nucleus by making use of the nuclear magnetic resonance (NMR) phenomena.
As an apparatus for examining the internal organs and tissues in a human body from the outside, the following apparatus have been widely used in the prior art:
X-ray apparatus (including computer tomography apparatus, i.e., a CT scanner); PA1 nuclear medical apparatus; and PA1 ultrasonic apparatus. PA1 .gamma.: a gyromagnetic ratio; and PA1 K: h/2.pi. (h: Planck constant).
Each of these apparatus has its respective advantages and disadvantages. For the purposes herein, only the brief descriptions necessary for the comparison with the NMR apparatus according to the invention will be set forth.
The prior art apparatus measure mainly the physical properties of a human body. More specifically, X-ray apparatus discriminates between human tissues and defines their shapes in accordance with the differences in the X-ray transmission characteristics among the human tissues so that a variety of diseases or conditions can be identified. On the other hand, ultrasonic apparatus finds the differences in acoustic impedance among the human tissues thereby to discriminate between tissues and to measure their shapes and motions. However, the nuclear medical apparatus measures the distribution of a radioactive isotope, or its compound, with which a human body has been dosed, and is thereby able to distinguish among the human tissues and to some extent determine the conditions of the respective tissues. Therefore, the nuclear medical apparatus aims mainly at gathering macroscopic information as to each tissue although it can attain biological information to some extent.
Thus, it is difficult for the conventional apparatus in examining a human body from the outside to gather biochemical information concerning the human tissues. More specifically, if the same X-ray transmission characteristic or acoustic impedance is exhibited, it is quite difficult to judge whether that particular tissue is biochemically normal or abnormal (e.g., cancerous). Especially X-ray apparatus has the disadvantage of the exposure of the tissues of the body to X-rays from the outside of the body, and the nuclear medical apparatus has the disadvantage of the exposure of the tissues of the body to .gamma.-rays from the inside of the body.
As is well-known in the art, for example, examinations are widely performed at present to sample cells and take their photographs by means of a gastro-camera. Some tissues and fluids, such as blood, can also be biochemically analyzed. According to this art, however, the tissues to be examined are restricted, and it is difficult to examine the tissues at the respective portions of a human body from the outside.
By the use of the principle of the NMR, on the contrary, it is possible to attain even biochemical information of the respective tissues in the human body from outside the body.
First of all, the principle of the NMR will be generally explained.
Atomic nuclei are composed of protons and neutrons. In some atomic nuclei, the protons and neutrons spin as a whole like a top, having a nuclear spin angular momentum denominated I. For example, the atomic nucleus (.sup.1 H) of hydrogen is composed of a single proton having a spin expressed by a spin quantum number of 1/2, as shown in FIG. 1(a). Here, since the proton P has a positive electric charge e+, as shown in FIG. 1(b), the phenomenon has the same electrical result as that produced when a current corresponding to the positive charge flows through a small coil in accordance with the spins of the nucleus so that a magnetic momentum .mu. is established. In other words, each hydrogen nucleus can be deemed as a small magnet.
As is diagrammatically shown in FIGS. 2(a) and 2(b), since a ferromagnetic element such as iron has its minute magnets oriented in the same direction (as shown in FIG. 2(a)), magnetization is observed as a whole. In the case of the aforementioned hydrogen, on the contrary, the directions of the respective magnetic momentums are at random (as shown in FIG. 2(b)) so that magnetization is not observed as a whole.
If a static magnetic field Ho in a direction Z is applied to the hydrogen, the respective atomic nuclei are rearranged in the direction Z of the static magnetic field Ho, i.e., the energy levels of the nuclei are quantized in the direction Z. This phenomena for the hydrogen nucleus is shown in FIG. 3(a). Since the spin quantum number of the hydrogen nucleus is 1/2, the energy level is divided into two levels, i.e., -1/2x.DELTA.E and +1/2x.DELTA.E, as shown in FIG. 3(b), but most of the nuclei are oriented in the direction Z corresponding to the energy level of +1/2x.DELTA.E. The energy difference between two energy levels is given by the following equation: EQU .DELTA.E=.gamma. KHo (1)
wherein:
If a force defined by .mu. x Ho is applied to each atomic nucleus by a static magnetic field Ho, the atomic nucleus performs a precession around the axis Z at an angular velocity defined by the following equation: EQU .omega.=.gamma.Ho (which is called the Lamor angular velocity) (2)
If electromagnetic waves (which are usually radio-frequency waves) having a frequency corresponding to the angular velocity are applied to the apparatus at that state, resonance takes place so that the atomic nuclei absorb the energy .gamma. KHo corresponding to the aforementioned energy difference .DELTA.E until they transit to the next higher energy level.
Even if several kinds of atomic nuclei having nuclear spin angular momentums are mixed, the gyromagnetic ratios .gamma. are different among the respective atomic nuclei so that the resonance frequencies are accordingly different thereamong. Therefore, the resonance of a specific atomic nucleus can be extracted. Moreover, if the intensity of the resonance is measured, the quantity of the atomic nuclei in the area in question can be obtained. After the resonance, on the other hand, the atomic nucleus, which was excited to a higher energy level, after the time determined by a time constant called a relaxation time has elapsed, returns to its lower energy level. The relaxation time, especially the spin-lattice relaxation time denoted at T1, is a time constant dependent upon the manner of combination of individual chemical compounds and is known to be very different for normal tissues and malignant tissues.
Although the foregoing description has been directed to the atomic nucleus (.sup.1 H) of hydrogen, other atomic nuclei having different nuclear spin angular momentums can be used to perform similar measurements. For example, in the usual chemical analysis, the atomic nucleus (.sup.19 F) of fluorine, the atomic nucleus (.sup.31 P) of phosphorus, the atomic nucleus (.sup.13 C) of carbon and so on are employed in addition to the aforementioned atomic nucleus (.sup.1 H) of hydrogen.
Thus, since the quantity of a specific atomic nucleus existing and its relaxation time can be measured by the NMR, chemical information as to a specific atomic nucleus existing in a substance can be obtained.
As an examining apparatus making use of NMR, there has been proposed an apparatus in which an NMR signal corresponding to each projection image of a body to be examined is measured in a number of directions with respect to the body on the basis of a similar principle to the so-called "first generation i.e., translate-rotate type, CT scanner" so that its intensity at each position of the body may be determined by the reconstructing method. ("Image Formation by Induced Local Interactions: Examples Employing Nuclear Magnetic Resonance," by P. C. Lauterbur, "NATURE" vol. 242, Mar. 16, 1973, pp. 190-191).
However, the apparatus proposed has a disadvantage in that it takes a long time to accomplish the measurements. Since it is difficult especially for the NMR to have a signal/noise (S/N) ratio as high as the X-ray, a measurement time larger than one order longer than that of the first generation CT scanner is required for accomplishing measurements which are worthy of examination.